Laser ablation of a target of source material is a known technique that is useful in the field of thin film deposition on a substrate. Typically, a pulsed laser beam is employed to produce a plume of ablated material in a process known as Pulsed Laser Deposition, or PLD.
Many different materials, such as simple oxides, ferroelectrics, and high T.sub.c superconductors, for example, have been deposited by pulsed laser deposition. Compared with traditional technologies such as electron beam deposition, chemical vapor deposition (CVD), and various sputtering techniques, for example, PLD generally allows better control over stoichiometry of the deposited material, provides capability for a more reactive environment, and produces evaporants with desired energies for improved film crystallinity. Secondary advantages of PLD include small target size (typically 1/2 inch in diameter), ability to grow multilayer heterostructures, introduction of dopants by plasma interaction, and the absence of inert sputtering gas. Despite these advantages, however, PLD has remained primarily a research tool. The evolution of PLD into a production compatible process has been hampered by two major problems: the lack of large area uniformity, and the rough surface morphology that results from particle inclusion.
The ablation plume produced by PLD is a result of adiabatic jet expansion, which is not Lambertian (i.e., not a Cos .THETA. distribution). The angular distribution of PLD ablation plumes follows a Cos.sup.n .THETA. law, where n can vary from about 2 to 11. This narrow angular distribution and the close target-to-substrate distance results in poor thickness uniformity of the deposited film. The best known data on large area uniformity that has been reported was obtained using off-center substrate rotation to achieve a 10% variation over a one inch square area. Other techniques include rastering the laser beam over a large area target or using an aperture to skim the ablation plume while moving the substrate in the x-y direction.
In addition to nonuniformity, the inclusion of particles in a thin film degrades the quality of the film and the performance of the end device. For example, particle inclusion causes light scattering in optical applications and can cause electrical shorts in ferroelectric thin film memory devices. The poor surface morphology produced by laser deposition is a problem associated with the intense interaction between laser and target, which causes splashing from mechanisms such as exfoliation, subsurface boiling, and repulsion of the surface molten layer by recoil pressure of the expanding ablative plasma. With the exception of a few materials, such as yttria stabilized zirconia (YSZ), SrTiO.sub.3, MgO, La.sub.0.5 Sr.sub.0.5 CoO.sub.3, and a few other materials, nearly all laser ablated materials suffer from this problem. Some materials, such as Ge, produce "particle-free"films if a molten target is used. Other techniques include the use of a mechanical velocity filter, synchronous pulses from a second laser, synchronous pulsed oxygen nozzle beams, and careful preparation of the target surface. All of these techniques, however, are only marginally successful in improving surface morphology of the deposited films. If pulsed laser deposition is to be successful in transitioning from the laboratory to large scale fabrication operations, techniques must be developed to eliminate particle inclusion and provide large area uniformity of the deposited film.